This invention relates to the oxidation of ammonium salts of sulfuric acid contained in aqueous media; and, more particularly, to a reductive combustion process which produces a combustion gas containing a divalent sulfur compound such as hydrogen sulfide and/or carbonyl sulfide, and further to the conversion of such divalent sulfur compound to other useful sulfur products.
Various industrial processes produce aqueous by-product streams comprising ammonium salts of sulfuric acid. For example, Ruest U.S. Pat. No. 4,524,077 and Hernandez U.S. Pat. No. 4,912,257 both describe processes for the preparation of 2-hydroxy-4-methylthiobutanoic acid (HMBA) by sulfuric acid hydrolysis of 2-hydroxy-4-methylthiobutanenitrile (HMBN). In each process, an aqueous hydrolyzate is produced comprising HMBA and ammonium bisulfate. In Ruest, the aqueous hydrolyzate is extracted with a substantially water-immiscible solvent for recovery of the product HMBA. Raffinate from the extraction is stripped for recovery of solvent, producing a bottoms fraction which comprises ammonium bisulfate. Depending on hydrolysis conditions, the raffinate stripper bottoms may also contain some ammonium sulfate or free sulfuric acid.
In Hernandez, the hydrolyzate is neutralized with ammonia, causing separation of an organic phase containing HMBA from an aqueous phase containing ammonium sulfate.
The by-product ammonium salt solutions produced in the Ruest and Hernandez process generally lack economic value, and must be disposed of in some manner. U.S. Pat. Nos. 5,498,790 and 5,670,128 describe processes for the regeneration of sulfuric acid from the by-product ammonium salt solutions, and recycle of the regenerated acid for further hydrolysis of HMBN to HMBA. Processes are known for the recovery of ammonium sulfate for use in fertilizers or other applications. However, such processes are complicated, and the market value of ammonium sulfate is generally not sufficient for recovery of the processing costs.
Other processes which produce by-product ammonium salts of sulfuric acid include the preparation of caprolactam and the preparation of methyl methacrylate. Sulfuric acid regeneration processes have been proposed for treating the by-product salt solutions in these instances as well. See, for example, U.S. Pat. Nos. 3,549,320 and 4,490,347 directed to the treatment of waste ammonium sulfate solution produced in the preparation of methyl methacrylate.
In all the sulfuric acid regeneration processes, a solution or slurry of by-product ammonium salt, or solid particulate salt, is introduced together with fuel into a combustion furnace wherein the salt is pyrolyzed to produce a combustion gas comprising sulfur dioxide, carbon dioxide, water vapor, nitrogen, excess oxygen, and typically oxides of nitrogen. Ammonia released from the salt is burned in the process to yield water vapor and nitrogen. After the gas stream has been cooled and cleaned, it is typically passed to a converter in which sulfur dioxide is catalytically converted to sulfur trioxide. Absorption of sulfur trioxide in sulfuric acid yields concentrated sulfuric acid which may be recycled or otherwise used or sold. Because the object of the sulfuric acid regeneration processes is to produce a gas stream containing both SO2 and O2 for further oxidation in the catalytic converter, all these processes introduce at least a slight excess of oxygen, typically in the form of air.
German Offlengungsschrift 197 54 562 A1 describes an alternative process for the recovery of sulfuric acid from sulfur-containing secondary products of a process for the preparation of HMBA. In that process, an aqueous mixture comprising ammonium sulfate or ammonium bisulfate is introduced into a combustion zone and burned to produce a combustion gas containing sulfur dioxide. The cooled combustion gas is contacted with a hydrogen peroxide solution to produce sulfuric acid. The acid produced can be used for the hydrolysis of HMBN to HMBA.
Certain of the known sulfuric acid regeneration processes use two stage combustion as a means to reduce the concentrations of oxides of nitrogen in the combustion gas leaving the combustion operation. Such operation is described, for example, in U.S. Pat. Nos. 5,498,790 and 5,670,128 wherein the first stage is operated with a slight deficiency of air so that nitrites, ammonia, and any amines contained in the sulfate feed solution are oxidized to nitrogen gas and carbon oxides but not to oxides of nitrogen. The partial combustion gas leaving the first stage contains unburned combustibles and carbon monoxide. To fully convert the carbon, hydrogen and sulfur content of combustibles and carbon monoxide to carbon dioxide, water, and SO2, oxidizing conditions are established in the second stage of the combustion by further injection of air. The sum of the air provided to the two stages is preferably sufficient to provide an oxygen content between about 0.5% and about 5% in the gas leaving the combustion chamber.
BASF DE Offenlegungsschrift 41 01 497 describes a process for thermal workup of waste water containing ammonium sulfate by metering the waste water through a nozzle/burner system, consisting of a burner and a centrally located atomization nozzle, into an adiabatic combustion chamber using a supporting fuel and a suitable oxidizing agent, typically air. Two stage combustion is carried out first at reducing, then at oxidizing conditions. In the first stage, aqueous ammonium sulfate is burned reductively to produce hydrogen sulfide in a combustible mixture of reducing gases. Nitrogen bound in the ammonium sulfate is mainly reacted to nitrogen (N2) and not to nitrogen oxides. In the second stage additional air is added to facilitate complete combustion of the reduced species in the offgas, H2S being converted to SO2. The process is said to be characterized by the fact that with reducing and/or oxidizing reaction conditions at a temperature between 600xc2x0 C. and 2000xc2x0 C., particularly at temperatures between 900xc2x0 C. and 1100xc2x0 C., the nitrogen bound in the ammonium sulfate is mainly reacted to N2 with the simultaneous recovery of gaseous sulfur compounds (SO2, SO3, COS, H2S, etc.). In Example 1 of the BASF patent, a 10% by weight ammonium sulfate solution is introduced through a burner with a centrally located atomization lance into an adiabatic, vertically placed combustion chamber, in which reducing conditions are established by appropriately controlling combustion. Natural gas is used as the fuel with 80% theoretical air, producing a combustion gas containing CO, H2, H2S, and zero NO. The flue gas is said to be conducted to a gas workup in which the gaseous sulfur compounds (COS and H2S) are separated. The reference further reports that the purified sulfur-containing gases can be carried through a working unit in a subsequent process, e.g., reaction to elemental sulfur in a Claus unit. The remaining gas is used for the production of steam by the under-firing of a boiler or can be used for heating purposes by subsequent combustion. Other examples describe oxidative combustion of the ammonium sulfate solution.
The ammonium sulfate solutions used in the process of DE 41 01 497 are relatively dilute, thereby requiring a substantial energy input for vaporization of water, and producing a combustion gas in which sulfur-bearing gases are diluted with water vapor and other products of combustion. Although Example 1 of the ""497 publication states that the hydrogen sulfide produced in the reductive combustion can be converted to sulfur in a Claus unit, the hydrogen sulfide content of the combustion gas is relatively low. The ""497 publication does not describe measures to maximize either the hydrogen sulfide, carbon monoxide, or hydrogen content of the combustion gas.
In U.S. Pat. No. 4,208,390, Hirabayashi et al. describe a process for the recovery of ammonia and sulfur dioxide from an aqueous mixture containing an ammonium salt of sulfuric acid obtained as a by-product of the preparation of xcex5-caprolactam or cyclohexanone oxime. The by-product mixture comprises a roughly 50% by weight aqueous solution of ammonium bisulfate, and is reacted, in finely divided form, at a temperature of 700xc2x0 to 950xc2x0 C. with gases obtained from the combustion of a fuel and a controlled amount of oxygen, releasing ammonia and sulfur dioxide which are thereafter separated from the reaction mixture. The amount of oxygen is controlled to about 96% of the theoretical oxygen, i.e., the amount of oxygen that would be required to convert the nitrogen, sulfur, hydrogen and carbon contained in the by-product mixture and fuel into, respectively, N2, SO2, H2O, and CO2. The reaction gas discharged from the combustion furnace was reported to contain ammonia and 95.6% of the SO2 that theoretically could have been formed, 4.6% of the sulfur contained in the feed mixture having been converted to SO3.
Diaz-Bossio et al., xe2x80x9cReductive Decomposition of Calcium Sulfate Utilizing Carbon Monoxide and Hydrogen, Chem. Eng. Sci., Vol. 40, No. 3, pp. 319-324, describe thermal reduction of calcium sulfate to calcium sulfide using carbon monoxide and hydrogen produced by reforming methane. Other references disclose that sodium sulfate can be reduced to sodium sulfide by use of hydrogen, Birk et al. xe2x80x9cHydrogen Reduction of Alkali Sulfate,xe2x80x9d Ind. Eng. Chem. Proc. Des. Dev., 10(1), pp. 7-13 (1971), Nyman and O""Brien, xe2x80x9cCatalytic Reduction of Sodium Sulfate,xe2x80x9d Ind. Eng. Chem., 39(8), pp. 1021-1023 (1947), and White and White, xe2x80x9cManufacture of Sodium Sulfide,xe2x80x9d Ind. Eng. Chem., 28(1), pp. 244-246; or with carbon monoxide, Li and Heinigen, xe2x80x9cKinetics of Sodium Sulfate Reduction in the Solid State by Carbon Monoxide,xe2x80x9d Chem. Eng. Sci., 43(8), pp. 2079-2085 (1988) and Zou et al., xe2x80x9cCarbon Monoxide Reduction of Sodium Sulfate Mixed with Sodium Titanate,xe2x80x9d Can. J. Chem. Eng., pp. 892-893 (1993).
Among the several objects of the present invention may be noted the provision of a process for the destruction of aqueous by-products containing ammonium salts of sulfuric acid, in particular such by-products as are produced in the manufacture of methionine and HMBA; the provision of such a process in which reductive combustion is carried out at relatively high energy efficiency; the provision of such a process which converts sulfur contained in the aqueous by-products to a divalent sulfur compound; the provision of such a process which yields divalent sulfur compounds that can be converted to other useful sulfur-bearing products; the provision of such a process which produces a combustion gas containing significant concentrations of hydrogen sulfide and/or other divalent sulfur compounds; the provision of such a process which yields sulfur-bearing products that can be used as raw materials in the preparation of methionine or HMBA, and in particular raw materials that can be recycled for use in the methionine or HMBA manufacturing process.
Briefly, therefore, the present invention is directed to a process for destruction of an aqueous mixture containing an ammonium salt of sulfuric acid. An aqueous feed mixture containing ammonium ion in a proportion of at least 3% by weight and a sulfur-bearing component selected from among sulfate ion, bisulfate ion and sulfuric acid in a total proportion of at least about 10% by weight, expressed as SO4xe2x88x922, is introduced into a reductive combustion zone, thereby producing a combustion gas containing a divalent sulfur compound in a concentration of at least about 4500 ppm by volume, dry gas basis.
The invention is further directed to a process for recovery of a divalent sulfur compound from an aqueous mixture containing an ammonium salt of sulfuric acid. An aqueous feed mixture comprising an ammonium salt of sulfuric acid is introduced along with an oxygen-containing gas into a reductive combustion zone thereby producing a combustion gas containing hydrogen sulfide; and hydrogen sulfide produced in the combustion gas is converted to a divalent sulfur compound.
The invention is further directed to a process for preparation of an a-substituted carboxylic acid compound selected from methionine and 2-hydroxy-4-methylthiobutanoic acid. Methyl mercaptan is reacted with acrolein to produce 3-methylthiopropanal, and 3-methylthiopropanal is reacted with hydrogen cyanide to produce 2-hydroxy-4-methylthiobutanenitrile. Optionally, 2-hydroxy-4-methylthiobutanenitrile is reacted with ammonia to produce 2-amino-4-methylthiobutanenitrile. A hydrolysis substrate selected from among 2-hydroxy-4-methylthiobutanenitrile and 2-amino-4-methylthiobutanenitrile is contacted with a hydrolyzing acid selected from among sulfuric acid, ammonium bisulfate and mixtures thereof to produce an xcex1-substituted carboxylic acid product selected from 2-hydroxy-4-methylthiobutanoic acid and methionine and a by-product ammonium salt of sulfuric acid. Carboxylic acid product and ammonium salt are separated from the hydrolyzate. An aqueous feed mixture containing the separated ammonium salt is introduced into a reductive combustion zone thereby producing a combustion gas containing hydrogen sulfide. Hydrogen sulfide produced in the combustion gas is converted to methyl mercaptan; and methyl mercaptan is recycled for reaction with acrolein.
The invention is further directed to a process for the preparation of an a-substituted carboxylic acid compound selected from methionine and 2-hydroxy-4-methylthiobutanoic acid. Methyl mercaptan is reacted with acrolein to produce 3-methylthiopropanal. 3-Methylthiopropanal is reacted with hydrogen cyanide to produce 2-hydroxy-4-methylthiobutanenitrile. Optionally, 2-hydroxy-4-methylthiobutanenitrile is reacted with ammonia to produce 2-amino-4-methylthiobutanenitrile. A hydrolysis substrate selected from 2-hydroxy-4-methylthiobutanenitrile and 2-amino-4-methylthiobutanenitrile is contacted with a hydrolyzing acid selected from sulfuric acid, ammonium bisulfate and mixtures thereof to produce an xcex1-substituted carboxylic acid product selected from 2-hydroxy-4-methylthiobutanoic acid and methionine and a by-product ammonium salt of sulfuric acid. Carboxylic acid product and ammonium salt are separated from the hydrolyzate and an aqueous feed mixture containing the ammonium salt is introduced into a reductive combustion zone thereby producing a combustion gas containing hydrogen sulfide. The combustion gas containing hydrogen sulfide is contacted with a further supply of oxygen containing gas in a secondary combustion zone thereby producing a secondary combustion gas comprising sulfur dioxide. Sulfur dioxide produced in the secondary combustion zone is contacted with oxygen over an oxidation catalyst to produce a conversion gas containing sulfur trioxide; and sulfur trioxide is contacted with sulfuric acid in an SO3 absorption zone to produce an absorption acid containing incremental sulfuric acid produced in a liquid phase on absorption.
The invention is further directed to a process for the preparation of an xcex1-substituted carboxylic acid compound selected from methionine and 2-hydroxy-4-methylthiobutanoic acid. Methyl mercaptan is reacted with acrolein to produce 3-methylthiopropanal. 3-Methylthiopropanal is reacted with hydrogen cyanide to produce 2-hydroxy-4-methylthiobutanenitrile. Optionally, 2-hydroxy-4-methylthiobutanenitrile is reacted with ammonia to produce 2-amino-4-methylthiobutanenitrile. A hydrolysis substrate selected from 2-hydroxy-4-methylthiobutanenitrile and 2-amino-4-methylthiobutanenitrile is contacted with a hydrolyzing acid selected from sulfuric acid, ammonium bisulfate and mixtures thereof to produce an xcex1-substituted carboxylic acid product selected from 2-hydroxy-4-methylthiobutanoic acid and methionine and a by-product ammonium salt of sulfuric acid. Carboxylic acid product and ammonium salt are separated from the hydrolyzate and an aqueous feed mixture containing the ammonium salt is introduced into a reductive combustion zone thereby producing a combustion gas containing hydrogen sulfide. The combustion gas containing hydrogen sulfide is contacted with a further supply of oxygen containing gas in a secondary combustion zone thereby producing a secondary combustion gas comprising sulfur dioxide. Sulfur dioxide produced in the secondary combustion zone is contacted with hydrogen peroxide to produce sulfuric acid.
The invention is still further directed to a process for the preparation of an xcex1-substituted carboxylic acid compound selected from methionine and 2-hydroxy-4-methylthiobutanoic acid. Methyl mercaptan is reacted with acrolein to produce 3-methylthiopropanal. 3-methylthiopropanal is reacted with hydrogen cyanide to produce 2-hydroxy-4-methylthiobutanenitrile. Optionally, 2-hydroxy-4-methylthiobutanenitrile is reacted with ammonia to produce 2-amino-4-methylthiobutanenitrile. A hydrolysis substrate selected from 2-hydroxy-4-methylthiobutanenitrile and 2-amino-4-methylthiobutanenitrile is contacted with a hydrolyzing acid selected from sulfuric acid, ammonium bisulfate and mixtures thereof to produce an xcex1-substituted carboxylic acid product selected from 2-hydroxy-4-methylthiobutanoic acid and methionine and a by-product ammonium salt of sulfuric acid. Carboxylic acid product and ammonium salt are separated from the hydrolyzate and an aqueous feed mixture containing the ammonium salt is introduced into an incinerator. The ammonium salt is oxidized in the incinerator to produce an oxidative combustion gas comprising sulfur dioxide. The sulfur dioxide produced in the incinerator is contacted with a hydrocarbon gas comprising methane, thereby producing a process gas comprising carbon oxides, hydrogen sulfide and hydrogen. The carbon oxides, hydrogen sulfide and hydrogen contained in the process gas are passed through a catalytic reaction zone to form methyl mercaptan, which is recycled for reaction with acrolein.
Other objects and features will be in part apparent and in part pointed out hereinafter.